Ronald Maddison, 20, died in 1953 after having 200 milligrammes of the deadly nerve agent sarin dropped on to a piece of uniform material which was wrapped around his arm.

His family claim he and other military personnel at the Pordon Down military base in southwest England were duped into taking part in what they thought were harmless experiments to find a cure for the common cold.

The Ministry of Defence (MOD) insisted all the servicemen tested at Porton Down were told beforehand they were taking part in nerve gas experiments.

After 64 days of the inquest, the jury of six men and four women took just five hours to return a verdict of unlawful killing.

They decided that the cause of Maddison's death was "application of a nerve agent in a non-therapeutic experiment".

Maddison was one of hundreds of human volunteers involved in tests from 1939 to 1989 and lawyers believe the verdict could open the door for other service personnel to sue the MOD.

"We will be seeking legal advice on whether we wish to consider a judicial review," said an MOD spokesman.

"We don't believe the verdict today has implications for other volunteers. However, we will consider the implications," the spokesman said.

A first inquest in 1953, held behind closed doors "for reasons of national security", ruled that Leading Aircraftman Maddison's death was caused by misadventure.

In 2002, the High Court quashed that verdict and ordered that a new inquest should be held.

It was reopened following a two-million-pound (2.9-million-euro, 3.7-million-dollar) inquiry by police into chemical and biological agent research at Porton Down during the 50 years from 1939.

Operation Antler was launched in July 1999 after ex-serviceman Gordon Bell alleged he had been duped into taking part in tests.

All rights reserved. © 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.